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waht's a rev limiter for?

Thrasher

Well-Known Member
Joined
May 13, 2005
Messages
520
Location
Vancouver, Washington
Car(s)
1976 BMW 2002
Well duh it's for limiting the revs :p but why are they needed and what benefit do they serve


Also, how do they work and why do they make that popping/studdering sound
 
The rev limiter on a car is there to prevent over-reving, which will destroy the engine.
Simply removing the rev limiter, without making any other modifications to a car, will do next to nothing. In some cases, being able to rev the engine higher will yield slightly better performance, often at the cost of more engine wear...

-http://www.lunaticfringe.org/vwfox/mod/rev-limiter.html
 
rev limiter is part of your car's ECU that cuts off the fule supply at programed RPM to stop driver from over-reving by late gear changes, which usually results in bent values, cracked cylinder heads ect

however it only stops the engine from self-over reving, it can not prevent mechanical over-rev (ex: shift down to 2nd from 4th at 80mph)
 
Well, any car that was built within the last 10 years is going to shut itself down in a situation like that, right?

Buba
 
^you can shut down what you want, but if your car is moving and the clutch is locked, the engine will rev

the biggest problems with high revs is the heat, which occurs due to the combustion and the friction. but if you're not on the throttle, there is no fuel and so no combustion, keeping the heat to a minimum

and since a car is 4-stroke, the engine is still lubricated, so the chances you blow or block your engine when downshifting is pretty minimal, but i wouldn't do it all the time
 
caus the fuel is cut off, so it misses some explosions, what makes the sound fall away

but if you miss an explosion, your rpm will drop, but you're still on the throttle, so once below the limit, it will inject fuell again, rising rpm untill you're at the limit again, that's why most of the time you hear several pops when you hit the rev limiter
 
^^^
The popping sound is unburnt fuel making it past the combustion chamber then igniting when it comes in contact with a hot exhaust and oxygen.

Limiters are there to stop over-revving as has been already said. Engines' internals are designed to a certain stress and then a safety factor is applied. Where the limiter comes in, is to stop they added stress of changing the direction of the piston at such high speeds.

Also

rev limiter is part of your car's ECU that cuts off the fule supply at programed RPM to stop driver from over-reving by late gear changes, which usually results in bent values, cracked cylinder heads ect
 
Leppy said:
^^^
The popping sound is unburnt fuel making it past the combustion chamber then igniting when it comes in contact with a hot exhaust and oxygen.

no, that's a fart like we say

that mainly happens on turbo cars, air is compressed, so you need more fuel, but when shifting, the turbo slows down and the pressure falls away, but the same amount of fuel is injected (caus your injection/carb doesn't know the speed of the turbo spinning), so you have to much, this gets into the exhause and explodes there spontaneously
 
mmm It happens alot at my house. I live on a massive hill and people just go down the hill in 3rd and let the engine braking keep them at a decent pace the whole time the car is popping the way down the hill.

I haven't seen a turbo car do it yet.
 
bone said:
that mainly happens on turbo cars, air is compressed, so you need more fuel, but when shifting, the turbo slows down and the pressure falls away, but the same amount of fuel is injected (caus your injection/carb doesn't know the speed of the turbo spinning), so you have to much, this gets into the exhause and explodes there spontaneously
http://karne.pl/en/indictment.html
Huh? It just runs rich for an injection cycle or two, what's all this about turbine speed?
 
Last edited:
adrianpike said:
bone said:
that mainly happens on turbo cars, air is compressed, so you need more fuel, but when shifting, the turbo slows down and the pressure falls away, but the same amount of fuel is injected (caus your injection/carb doesn't know the speed of the turbo spinning), so you have to much, this gets into the exhause and explodes there spontaneously

Huh? It just runs rich for an injection cycle or two, what's all this about turbine speed?

on a normal engine it shouldn't be happening all that much, it can occur, but if it happens often, you probably need to remap your timing, while on a turbo car it can happen with each shift

leppy said:
mmm It happens alot at my house. I live on a massive hill and people just go down the hill in 3rd and let the engine braking keep them at a decent pace the whole time the car is popping the way down the hill.

I haven't seen a turbo car do it yet.

there you have it, a massive hill going down

they probably have the engine in to high RPM while braking (close to redline), and you allways have a very little petrol injected to keep it from stalling, but even this stops when the rev limiter kicks in, resulting in the popping sound (probably, caus where i live there aren't that many hills and haven't heard the popping sound on a car engine braking)

petrol which explodes in the exhause realy doesn't make a "popping" sound, it makes a bang, which you still can hear 3 blocks away.
we are talking about 2 completely different things right now
 
oh. I just mainly hear it when Im watching the driving school at the local racetrack. I also heard it on the Fromula Drift cars when they came too.
 
bone said:
adrianpike said:
bone said:
that mainly happens on turbo cars, air is compressed, so you need more fuel, but when shifting, the turbo slows down and the pressure falls away, but the same amount of fuel is injected (caus your injection/carb doesn't know the speed of the turbo spinning), so you have to much, this gets into the exhause and explodes there spontaneously

Huh? It just runs rich for an injection cycle or two, what's all this about turbine speed?

on a normal engine it shouldn't be happening all that much, it can occur, but if it happens often, you probably need to remap your timing, while on a turbo car it can happen with each shift

leppy said:
mmm It happens alot at my house. I live on a massive hill and people just go down the hill in 3rd and let the engine braking keep them at a decent pace the whole time the car is popping the way down the hill.

I haven't seen a turbo car do it yet.

there you have it, a massive hill going down

they probably have the engine in to high RPM while braking (close to redline), and you allways have a very little petrol injected to keep it from stalling, but even this stops when the rev limiter kicks in, resulting in the popping sound (probably, caus where i live there aren't that many hills and haven't heard the popping sound on a car engine braking)

petrol which explodes in the exhause realy doesn't make a "popping" sound, it makes a bang, which you still can hear 3 blocks away.
we are talking about 2 completely different things right now

Nah they are not on the limiter and I'm not talking about a backfire either. Go watch the episode where JC drives the F50 and how he marvels at the amount of unburnt fuel dumped into the exhaust on overrun.
 
A rev limiter is just a set rpm that the manufacturer determined was optimal for engine wear/performance. Sure, you're free to rev higher than that but most stock cams aren't designed to make high end power. Most street cars make low/mid-range power. Sports cars have high end, but that power quickly drops about 500rpm below redline.

If you want your engine to make power above redline, you should buy a very aggressive cam with high lift and duration. Don't forget stiffer valve springs! Otherwise, your stock springs won't be strong enough to close the valve on each stroke.
 
Leppy said:
Nah they are not on the limiter and I'm not talking about a backfire either. Go watch the episode where JC drives the F50 and how he marvels at the amount of unburnt fuel dumped into the exhaust on overrun.

true, but the F50 has a high performance engine. and 1 thing they don't want, is a lean condition, so they just inject to much.
ferrari can afford to do that :x

you may not compare a performance engine to a regular one
 
There are two forms of rev limiter, fuel cut and spark cut.
-With a spark cut, unburnt fuel will be dumped into the exhaust.
-With a fuel cut, unburnt fuel will *not* be dumped into the exhuast.

Spark cut = flames, backfire, pedestrians running.
Fuel cut = huge hesitation.

And I like comparing a performance engine to a regular one... Apples and Oranges are good fruit.
 
A friend borrowed the missus' Puma; put the car into second rather than 4th, bounced the engine off the limiter and threw a rod destroying the engine.

not good really.

Ford wanted ?4000 for a new engine, luckly I found a scrap puma with rear shunt and 19000 miles on the clock. Only ?400 for that engine.
 
Repeated popping while coasting IS fuel exploding in the exhaust.

Take a, oh i don't know, a peugeot 205 gti, for instance. The ECU is programmed so when you lift off the throttle at over 1600rpm or so, the fuel supply cuts (to save fuel). If you're coasting with a hot engine you may get a pop or two, but nothing notable.

But if you just apply "neutral" throttle, enough to nullify the engine braking, then you're treated to an orchestral series of "pop" sounds, akin to an F2 rally car.

Caburetted cars don't have any intelligent fuel supply control, so they are much more prone to this (and also to a proper backfire). Just cast your mind back to a hard tuned escort.

Highly strung turbocharged cars will puff visible clouds of fuel out of the exhaust between upshifts (remember the 400hp evo on top gear?!), which can/will ignite when the exhaust gets hot enough, giving you the classic turbo backfire.

The rev limiter is a handy device, it tries to stop you getting too much engine velocity.

When you rev an engine really hard, the pistons have to accelerate extremely fast, causing the conrods to stretch at the top of every stroke. Most road cars have heavy pistons with long skirts, to make them more mechanically economical, but it also means they're very heavy. When they're pulling 5000G's or what-have-you, the conrods can snap.

Of course, you could also get bent-valves from valve-float, cavitation in the cooling system, oil starvation.

One thing you do know though, is that engineers specify the rev-limiter to come on at a "safe" engine speed. As long as your engine is in good condition you should be able to smack into the limiter with impunity, without worrying about blowing the car up.
 
What about the Jag diesel JC drove at the Ring. He mis-shifted into a lower gear and the car went into limp mode BUT it was still drivable.
 
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